Introduce ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time UMA.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 28d90ca0cba04971f8185933e98c46b9baf60fd6..93f96d0d4fbe85cd7514eb001a87161e0d9eccd8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -49554,6 +49554,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time"
+    units="ms">
+  <owner>horo@chromium.org</owner>
+  <summary>
+    The time taken to find a Service Worker registration and wait for a worker
+    started to send a FetchEvent request for a main frame HTML file. The

[falken, 2016/06/07 07:33:48]

"HTML file" -> "resource" may be more usual wording.

[horo, 2016/06/07 07:54:59]

Done.

+    measurement starts when the browser process recieves the request and ends
+    when it finds a running worker or it successfully starts a worker. The
+    metric is not recorded in the following situations: 1) The worker was not
+    activated yet. This is intended to avoid including the time for the
+    activation. 2) The worker was started for the fetch AND DevTools was
+    attached during startup. This is intended to avoid including the time for
+    debugging. 3) The request is for New Tab Page.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ActivateEvent.Time" units="ms">
   <owner>shimazu@chromium.org</owner>
   <summary>
@@ -97058,6 +97074,17 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="ServiceWorker.StartWorker.Time"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="ServiceWorker.WorkerPreparationMode">
+  <suffix name="StartWorkerDuringStartup" label="StartWorkerDuringStartup"/>
+  <suffix name="StartWorkerNewProcess" label="StartWorkerNewProcess"/>
+  <suffix name="StartWorkerExistingProcess" label="StartWorkerExistingProcess"/>
+  <suffix name="StartingWorker" label="StartingWorker"/>
+  <suffix name="RunningWorker" label="RunningWorker"/>
+  <suffix name="StoppingWorker" label="StoppingWorker"/>

[falken, 2016/06/07 07:33:48]

Consider making these labels more description: they're appended to the
description of the affected histogram.

Suggestions:
"StartingWorker means the activated worker was already starting up."
"RunningWorker means the activated worker was already running."
"StoppingWorker means the activated worker was stopping."

[horo, 2016/06/07 07:54:59]

Done.

+  <affected-histogram
+      name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="ServiceWorkerCache.Cache" separator=".">
   <suffix name="Batch"
       label="The time to perform the 'batch' operation on a ServiceWorker